Polymerizable liquid crystal materials are known for the preparation of anisotropic polymer films with uniform orientation. These films are usually prepared by coating a thin layer of a polymerizable liquid crystal mixture onto a substrate, aligning the mixture into uniform orientation and polymerizing the mixture. The orientation of the film can be planar, i.e. where the liquid crystal molecules are oriented substantially parallel to the layer, homeotropic (rectangular or perpendicular to the layer) or tilted.
Such optical films are described, for example, in EP 0 940 707 B1, EP 0 888 565 B1 and GB 2 329 393 B1.
Polymerisable liquid crystal (LC) materials, while stable at room temperature, can degrade when subjected to increased temperatures. For example, when heated for a period of time the optical properties such as dispersion or retardance decreases and as such, the performance of the optical film degrades over time. This can be attributed, in particular, to a low degree of polymerisation and a corresponding high content of residual free radicals in the polymer, polymer shrinkage, and/or thermo-oxidative degradation.
A high degree of polymerisation can be, i.a., influenced by the choice of the utilized photoinitiator. In this regard, Nie et al. describe in JOURNAL OF APPLIED POLYMER SCIENCE, 123, 2, 2012, 725-731; the synthesis and photopolymerisation kinetics of suitable oxime ester photoinitiators. In addition to this, JP 5054456 B2 describes polymerisable liquid crystal (LC) materials comprising one or more direactive mesogenic compounds and the commercially available photoinitiators Oxe02 available from by Ciba and N-1919 (T) available from Adeka. However, polymerisable liquid crystal (LC) materials comprising one or more direactive mesogenic compounds and one or monoreactive mesogenic compounds are not disclosed.
In particular, the desired properties of an optical retardation film, like, e.g., uniform alignment of the mesogenic compounds, film structure, film adhesion, temperature stability and optical performance, are highly dependent from the composition of the polymerisable liquid crystal material especially concerning the ratio and choice of mono- and direactive mesogenic compounds.
For example, polymer shrinkage, which is a decrease in thickness of the optical film, reduces the retardance of the passing light in accordance to R=dΔn, wherein R is the retardance, d is the thickness of the birefringent film, Δn is the birefringence.
As commonly known, the polymer shrinkage can be reduced by utilizing polymerisable compounds having more than one polymerizable group, e.g. di- or multireactive compounds, and therefore capable of forming a more crosslinked and more rigid polymer. However, again, the desired properties of an optical retardation film are highly dependent from the composition of the polymerisable liquid crystal material. In this regard, one possible way to adjust the alignment profile in the direction perpendicular to the film plane is the appropriate selection of the ratio of monoreactive mesogenic compounds, i.e., compounds with one polymerizable group, and direactive mesogenic compounds, i.e. compounds with two polymerizable groups. In addition, low diacrylate content RM films are highly suitable for applications where good adhesion of the RM film to the substrate is important. However, as stated above, in low diacrylate content RM films often the optical retardation drops significantly especially due to polymer shrinkage.
Thermo-oxidative degradation is the breakdown of a polymer network catalysed by oxidation at high temperatures. As commonly known, antioxidant additives, or short antioxidants, can be used to reduce the thermo-oxidative degradation of polymers when subjected to increased temperatures. This is especially important when optical films are utilized for an in-cell application due to the high temperatures. In particular, the optical film has to endure when annealing the polyimide layer in the LC cell. In this regard, the documents WO 2009/86911 A1 and JP 5354238 B1 describe polymerisable liquid crystal (LC) materials comprising the commercially available antioxidant Irganox® 1076.
All of the above-described materials have distinct disadvantages, such as, the thermal durability of the resulting polymer films is still not high enough, their transparency to VIS-light is limited, they require the utilization of further additives, or their application bandwidth is limited, due to the utilized LC material.